This invention relates to centrifuges and more particularly, relates to automated centrifuges.
The oil industry has developed several methods for measuring capillary pressure and relative permeability of reservoir core samples using centrifuges. Capillary pressure and relative permeability are important properties for describing the flow of fluids in porous media and are generally needed in the reservoir engineering of an oil field. These properties help the reservoir engineer determine, for example, the productivity of a reservoir, the total reserves, and the likelihood of success for various oil recovery processes, such as water flooding or carbon dioxide flooding.
One of the preferred methods for measuring capillary pressure is the centrifuge method of Hassler and Brunner (Hassler, G. L. and Brunner, E., "Measurement of Capillary Pressures in Small Core Samples", Trans. AIME 1945, Vol. 160, pp 114). Similarly, a preferred method for measuring relative permeability is the centrifuge method of Hagoort (Hagoort, J., "Oil Recovery by Gravity Drainage", Soc. Pet. Eng. J., 1980, Vol. 20, p. 139).
These methods have the advantage of much greater speed compared to other methods for measurement and are amenable to automation. For both centrifuge methods, the core samples are mounted in special holders having glass collection tubes to allow for monitoring the production of fluid from the core samples. The cores are centrifuged and the effluent fluids from the samples are collected in the tubes. A strobed light source is used to determine the amounts of fluids collecting in the glass collection tubes.
Measuring the capillary pressure with the method of Hassler and Brunner requires increasing the speed of the centrifuge in "steps" or increments and measuring the amount of fluid produced from the core sample when all flow has ceased for that step (i.e. centrifuge speed) before increasing the centrifuge speed to the next "step". Measuring the relative permeability with the method of Hagoort requires running the centrifuge at a single speed, which is high enough to overwhelm capillary pressure effects, and measuring the amount of fluid produced from the core sample as a function of time.
In general, however, the capillary pressure or relative permeability curve determined by the prior art in a centrifuge experiment use effluent data alone. Since no measurements are made of the fluid saturations inside the core, various assumptions must be made concerning boundary conditions, uniformity of the displacement of the fluid, and homogeneity of the core. These assumptions may not always be valid, leading to inaccurate and unreliable results. However, even if these assumptions are valid, capillary pressure and relative permeability influence the measurement of each other.
The Hassler and Brunner method for measuring capillary pressure is confined to a drainage mode of flow for a water-wet core initially filled with a wetting fluid which is then invaded by a non-wetting fluid, i.e., oil invading a water-wet core. The method of Hassler and Brunner is not useful when a wetting fluid invades a water-wet core containing a non-wetting fluid as the equilibrium level of production of the non-wetting fluid is dependent upon imbibition and not centrifuge speed. However, such measurements are needed in order to design waterflood recovery methods where the invading fluid is wetting.
These and other limitations and disadvantages of the prior ar are overcome by the present invention however, and improved methods and apparatus are provided for centrifuging core specimens that are capable of determining fluid saturations inside the core specimen.